Cordierite, (2MgO . 2Al.sub.2 O.sub.3 . 5SiO.sub.2) and cordierite-like ceramic materials have been known and used for many years in the fabrication of thermal shock resistant cooking utensils . Generally, the prior art has made use of commercial clays of more or less plasticity combined with less plastic materials to provide the required constituents to develop a body near the stoichiometric equivalent of cordierite.
Most of the compositions in the prior art result in a body having high firing temperatures (1300.degree. C. to 1410.degree. C.), short firing range and poor wet working characteristics.
It is therefore the purpose of the present invention to diminish these problems by substituting common and uncommon mineral materials. In addition to the usual commercial, more or less plastic clay of reasonable purity, alumina or other source of Al.sub.2 O.sub.3, use is made of the slightly plastic, clay like, mineral of low tenacity, saponite (9MgO . Al.sub.2 O.sub.3 . 1OSiO.sub.2 . 15-16 H.sub.2 O, Dana's Textbook of Mineralogy, 7th ed.) or an unusual, hydrothermal, high magnesium, plastic clay of good tenacity known to occur in veins associated with said saponite. In addition to these, use is also made of the crystalline mineral tremolite in certain variations of the present composition.
The hydrothermal, high magnesium, plastic clay presents a problem of definition. Being of hydrothermal occurrence it is distinct in origin from the usual montmorillonite which is said to be the product of autogenous alteration or of weathering of clastic materials of volcanic origin or other clastic materials. It also differs from the usual montmorillonite in being higher in magnesium content (9 to 20% MgO as against 0 to 8.8% MgO for montmorillonite). Dry montmorillonite swells substantially with addition of water whereas the present high Mg. clay is similar to the usual ball clay in this respect.
The use of the term "cordierite like ceramic" as distinct from "cordierite ceramic" seems most suitable because the use of tremolite adds calcium which is not a constituent of the idealized cordierite crystal structure.
In spite of the CaO content, the use of tremolite is advantageous because the lath like form of the ground tremolite particles contributes to wet workability and possibly to firing stability.
At this writing it isn't known by the applicant how much of the CaO is in the form of inter-crystalline glass, mixed crystal, or substitutional in the cordierite lattice. But the thermal expansion characteristics remain suitable for thermal shock resistant cookware.
It is therefore the purpose of this invention to compound a ceramic body composition of such low thermal expansion characteristics that it is suitable for use in thermal shock resistant cookware.
It is another object of this invention to provide an unfired composition that has enhanced characteristics of tenacity and plasticity so as to facilitate fabrication in the moist state.
It is another object of this invention to provide a composition that has good dimension stability under drying and firing conditions.
It is another object of this invention to provide a composition that has a low firing temperature.
It is another object of this invention to provide a composition that has a firing range of sufficient scope that delicate temperature controls are not essential.
It is another object of this invention to provide a composition that has rapid drying characteristics.
These and other objects of this invention will be apparent to one skilled in the art from a total consideration of the disclosure.